Calcium-dependent cleavage of striatal enriched tyrosine phosphatase (STEP)

Striatal enriched phosphatase (STEP) is a family of protein tyrosine phosph atases enriched within the CNS. A member of this family, STEP61, is a membr ane-associated protein located in postsynaptic densities of striatal neuron s. In this study, we demonstrate that STEP61 is cleaved into smaller isofor ms. To clarify the mechanism of cleavage, STEP61 was transiently expressed in NT2/D1 neuronal precursor cells. Exposure of transfected cells to the ca lcium ionophore, A23187, or to thapsigargin resulted in the rapid cleavage of STEP61. Pretreatment with the calpain inhibitor, calpeptin, or EGTA prev ented proteolysis. One of the cleavage products has a relative molecular ma ss of 33 kDa (STEP33). A protein with the identical mobility is detected fo llowing calpain treatment of STEP61 fusion protein or postsynaptic densitie s purified from rat striatum. Exposure of primary neuronal cultures to glut amate also led to a significant increase in the concentration of a low mole cular weight form of STEP. Taken together, these results suggest that in re sponse to a rapid influx of calcium, STEP61 is proteolytically cleaved by c alpain, leading to the release of a smaller isoform. This model may explain the rapid appearance of STEP33 in response to transient hypoxia-ischemia i n the brain as cells attempt to counter the increase in tyrosine phosphoryl ation levels following neuronal insults.